The present invention has to do with wellhead equipment used in connection with pumping oil wells. More particularly, as previously stated, it relates to a composite pumping tree. By "composite" is meant that the functional components referred to herein as top and bottom connectors, shut-off valve, production blow-out preventer and flow tee are integrated into a single steel body. The term "pumping tree" is used to generically encompass the body or housing, its component openings, its axial fluid flow bore and the functional components if they are mounted in the body openings.
For many years, a typical conventional production wellhead assembly for a pumping well was as shown in FIG. 1 and comprised, from the bottom up: a flanged casing bowl attached to the well casing; a flanged tubing head having an internal hanger from which the well tubing string was suspended; a tubing head adapter having a flanged connection at its bottom end and a threaded connection of smaller diameter at its top end; a production blow-out preventer ("B.O.P.") body having top and bottom threaded connections and including side openings for receiving the B.O.P. ram components; a flow tee body having a threaded or flanged side opening for connecting with a flow line; and additional components (not shown), such as a polish rod stuffing box and the rotary drive assembly for rotating the well's rod string to power a downhole progressive cavity pump. (The overall assembly extending up from the top of the tubing head to the drive assembly is commonly referred to as the production christmas tree.)
The production christmas tree, as described in the previous paragraph, is designed for use in connection with a pumping well. Since the tree is subjected only to relatively low pressure in service, its parts are relatively thin-walled.
There are some pumping wells which, when shut in, can build up significant pressure at the wellhead due to the presence of gas in the produced fluid. When the rod string is pulled from the well, to service the downhole pump, it would be desirable to have a gate valve in place between the tubing head and the production tree, to provide a positive and reliable shut-off.
In the case of a naturally flowing well, where greater pressures would be expected, one or more high pressure shut-off valves are stacked between the tubing head and the flow tee and the production B.O.P. is usually not included.
A recent improvement in the production wellhead art is disclosed in Canadian Patent 2,197,584, issued Jul. 7, 1998 to the present applicant. More particularly this patent teaches integrating the tubing head adapter, B.O.P. and flow tee into a unitary structure or tree by forging or casting a single steel body or structure forming an axial vertical fluid flow bore and further comprising, from the bottom up:
a bottom connector or connection sized and designed to connect and seal with the top connection of the tubing head; PA1 a B.O.P. housing section forming side openings connecting with the axial bore, for receiving the ram components of a B.O.P.; PA1 a flow tee housing section forming a side opening and providing means for connecting with a flow line; and PA1 a top connector sized and designed to connect with the upper section of the christmas tree (typically with the stuffing box). PA1 the unitary tree is significantly shorter and stronger as a result of removing the threaded or flanged connections between the functional components. This is particularly useful in the case where a vibrating offset rotary drive assembly is mounted on the top end of the christmas tree for rotating the rod string of a downhole progressive cavity pump; and PA1 the thick side wall of the cylindrical body is amenable to drilling additional openings from the side, for example for insertion of an instrumentation string to measure bottom hole temperature or pressure. This feature permits the tree to be customized to meet the particular needs of a customer.
In a preferred form, the generally tubular '584 tree has a generally cylindrical outer configuration, thereby ensuring a relatively thick body side wall of consistent thickness.
The '584 tree has been characterized by certain advantages, more particularly: